Hepatocyte growth factor (HGF), also known as scatter factor (SF), because of its ability to disrupt colony formation in vitro, is a mesenchymally derived cytokine known to induce multiple pleiotropic responses in normal and neoplastic cells (Sonnenberg et al., J. Cell Biol. 123:223-235, 1993; Matsumato et al., Crit. Rev. Oncog. 3:27-54,1992; and Stoker et al., Nature 327:239-242, 1987). These responses are known to include proliferation in both epithelial and endothelial cells, dissociation of epithelial colonies into individual cells, stimulation of motility (motogenesis) of epithelial cells, cell survival, induction of cellular morphogenesis (Montesano et al., Cell 67:901-908, 1991), and promotion of invasion (Stella et al., Int. J. Biochem. Cell Biol. 12:1357-62, 1999 and Stuart et al., Int. J. Exp. Path. 81:17-30, 2000), all critical processes underlying metastasis. HGF has also been reported to promote angiogenesis (Bussolino et al., J. Cell Biol. 119:629-641, 1992). In addition, HGF plays a critical role in tissue regeneration, wound healing, and normal embryonic processes, all of which are dependent on both cell motility and proliferation.
HGF initiates these physiological processes through high affinity binding to its cognate receptor, the Met protein tyrosine kinase receptor, an identified protooncogene (Park et al., Proc. Natl. Acad. Sci. USA 84:6379-83, 1987 and Bottaro et al., Science 251:802-4, 1991). The mature form of Met consists of a highly glycosylated external α-subunit as well as a β-subunit with a large extracellular domain, a transmembrane segment and a cytoplasmic tyrosine kinase domain. Ligand engagement induces Met dimerization that results in an autophosphorylated activated receptor. Activation of Met promotes signal transduction cascades as defined by transphosphorylation of key cytoplasmic tyrosine residues responsible for recruiting multiple effector proteins (Furge et al., Oncogene 19:5582-9, 2000). These include the p85 subunit of the PI3-kinase, phospholipase Cγ (Gaul et al., Oncogene 19:1509-18, 2000), Grb2 and Shc adaptor proteins, the protein phosphatase SHP2 and Gab1. The latter adapter has emerged as the major downstream docking molecule that becomes tyrosine phosphorylated in response to ligand occupancy (Schaeper et al., J. Cell Biol. 149:1419-32, 2000; Bardelli, et al., Oncogene 18:1139-46, 1999 and Sachs et al., J. Cell Biol. 150:1375-84, 2000). Activation of other signaling molecules has been reported in HGF stimulated cells, most notably Ras, MAP kinases, STATs, ERK-1, -2 and FAK (Tanimura et al., Oncogene 17:57-65,1998; Lai et al., J. Biol. Chem. 275:7474-80 2000 and Furge et al., Oncogene 19:5582-9, 2000). The role of many of these signaling molecules has been well established in cell proliferation.
Met, also referred to as hepatocyte growth factor receptor (HGFR), is expressed predominantly in epithelial cells but has also been identified in endothelial cells, myoblasts, hematopoietic cells and motor neurons. Overexpression of HGF and activation of Met has been associated with the onset and progression in a number of different tumor types as well as in the promotion of metastatic disease. Initial evidence linking Met to cancer has been supported by the identification of kinase domain missense mutations, which predisposes individuals to papillary renal carcinomas (PRC) and hepatocellular carcinomas (HCC) (Lubensky et al., Amer. J. Pathology, 155:517-26, 1999). Mutated forms of Met have also been identified in ovarian cancer, childhood HCC, gastric carcinoma, head and neck squamous cell carcinoma, non-small cell lung carcinoma, colorectal metastasis (Christensen et al., Cancer Res., 63:7345-55, 2003; Lee et al., Oncogene, 19:4947-53, 2000 and Direnzo et al., Clin. Cancer Res., 1: 147-54, 1995). In addition, further evidence supporting the role of the Met in cancer is based on the overexpression of HGF and Met receptor in various tumors including thyroid, ovarian and pancreatic carcinomas. It has also been demonstrated to be amplified in liver metastases of colorectal carcinomas (Rong et al. Cancer Res. 55:1963-1970, 1995; Rong et al., Cancer Res. 53:5355-5360, 1993; Kenworthy et al., Br. J. Cancer 66:243-247, 1992 and Scarpino et al. J. Pathology 189:570-575, 1999). TPR-Met (an activated form similar to BCR/Abl in CML) has been described and identified in human gastric carcinoma (PNAS 88:4892-6, 1991). In patients with invasive breast carcinoma and in a recent study in non small cell lung cancer patients, expression of either the receptor or ligand is a predictor of decreased survival, further linking Met to tumor progression (Camp et al., Cancer 86:2259-65 1999 and Masuya et al., Br. J. Cancer, 90:1555-62, 2004). In general, most human tumors and tumor cell lines of mesenchymal origin inappropriately express HGFR and/or HGF.
Numerous experimental data support the role of HGF and Met in tumor invasion, growth, survival and progression ultimately leading to metastases. Preclinically, transgenic expression of HGF results in a metastatic phenotype (Takayama et al., PNAS, 94:701-6, 1997) and an amplified/overexpressed Met spontaneously transforms NIH-3T3 cells (Cooper et al., EMBO J., 5:2623-8, 1986).
Biological agents, such as ribozymes, antibodies and antisense RNA targeting either HGF or Met have been shown to inhibit tumorogenesis (Stabile et al., Gene Therapy, 11:325-35, 2004, Jiang et al., Clin. Cancer Res, 9:4274-81, 2003 and Genentech U.S. Pat. No. 6,214,344, 2001). Thus, selective, small molecule kinase modulators targeting Met are expected to have therapeutic potential for the treatment of cancers in which Met receptor activation plays a critical role in the development and progression of primary tumors and secondary metastases.